This project focuses on elucidating the mechanisms by which different amacrine subtypes are determined in the developing Xenopus retina. During the past grant period it was shown that different subtypes, as defined by their neurotransmitter phenotype, arise from unique subsets of embryonic precursors. This result calls into question the prevailing notion that lineage-independent developmental mechanisms are the sole determinants of cell fate. In the next grant period, 4 questions will be investigated: (1) whether clones that produce particular subtypes of amacrine cells are capable of changing fate to produce other types; (2) whether small clusters of cells of common subtype arise via a common precursor or through local inductions; (3) whether the expression of two recently identified Xenopus transcription factors, XlPOU-2 and XDII-1, influence retinal cell fate of precursor blastomeres; and (4) what signaling pathways do cleavage-stage blastomeres require to express retinal lineages. Techniques to be used include lineage analysis by tracer injection, blastomere transplantation and photoablation, mitotic blockade, and altering gene expression through injection of normal or mutant oligonucleotides or mRNA.